Add IPv6 configuration examples for Rocky Linux 9

[saulolimabrito]

Summary

Related to #2425. This PR targets rocky-9 because the new examples were
validated on Rocky Linux 9.8.

Add an IPv6 configuration section to the existing network configuration
guide, covering:

• connection-profile names versus interface names
• nmtui Automatic and Manual modes
• manual IPv6 address, prefix, gateway and DNS configuration using nmcli
• switching back to automatic configuration and clearing manual overrides
• SLAAC, Router Advertisements, DHCPv6 context and link-local addresses
• address/route inspection, IPv6 ping and DNS verification
• documentation-only addresses and remote-connection reactivation behavior

Add the requested IPv6 and nmtui tags, retain existing contributors and
tested versions, and add the contributor and the version used for this work.

Validation

Test environment: Rocky Linux 9.8, NetworkManager/nmtui 1.54.3.

The lab used isolated client/router network namespaces and a private
NetworkManager/D-Bus instance with separate keyfiles and runtime state.
The router advertised a prefix and RDNSS; an IPv6 DNS server provided a
test AAAA record on a different subnet.

13 checks passed:

1. Manual IPv6 address configuration
2. Default route through the configured gateway
3. Gateway connectivity
4. Connectivity to another IPv6 subnet
5. DNS resolution using NetworkManager-generated resolver settings
6. nmtui Manual view (the Automatic view was also inspected)
7. Profile settings retained after reactivation
8. SLAAC address acquisition
9. Router Advertisement default route
10. RDNSS server acquisition
11. Automatic DNS resolution
12. Clearing manual settings when switching to automatic mode
13. IPv4 settings and connectivity preserved across IPv6 changes

The VM's host connection settings, routes, resolver configuration and
keyfiles matched their original state after cleanup. Public IPv6 Internet
connectivity and a stateful DHCPv6 server were not part of this lab.

Documentation checks:

• markdownlint-cli 0.38.0 with the repository configuration: passed
• CLI executed with Node.js 24.15.0
• YAML metadata validation: passed
• bash -n on all eight added shell blocks: passed
• Markdown rendering with tabbed content: new section is in the Rocky 9 tab
• Rocky 8 tab content is byte-for-byte unchanged
• git diff --check: passed

Author checklist

• Existing title/contributor metadata retained and contributor added
• New examples validated in the environment described above
• Typos, grammar and formatting reviewed

Rocky Documentation checklist

• First pass
• Technical review
• Editorial review
• Final approval

[sspencerwire]

Thank you very much @saulolimabrito

[sspencerwire]

@all-contributors add @saulolimabrito for content

[allcontributors]

@sspencerwire

I've put up a pull request to add @saulolimabrito! 🎉

[github-actions]

Test results for 248b26a:

Number of broken URLs: 0

URL,RESULT,FILENAME

[sspencerwire]

@saulolimabrito If you have experience trouble-shooting IPv6 connections, etc., we would love more IPv6 specific documentation. Thanks again.